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Caption

Particle accelerator tunnel construction. Workers drilling the tunnel for the super-proton- synchrotron (SPS) accelerator at CERN (the European particle physics laboratory) near Geneva, Switzerland. The tunnel is a 2 kilometre long ring. The SPS, which was operational from 1976, accelerated protons and antiprotons in the opposite direction and then focused them so that they collided. The subatomic particles produced by these collisions were then studied. It has more recently been used to accelerate high energy electrons and positrons for injection into the large electron-positron collider (LEP), and from 2007 will be used to accelerate protons and heavy ions, and inject them into the large hadron collider (LHC).